A simple login class to do basic protection on a folder.

login.class.php

<?php if ( ! defined('BASEPATH')) exit('No direct access allowed'); 
/**
* Handles a simple login scenario.
*
* @author Alex Garcia
* @copyright 2011
*/
class Login{
	
    //An array of values used in the system.
    //@var array
    public $config;

    //---------------------------------------------------------------------------------
    
    /**
    * Creates an instance of our Login object and sets the config values.
    */
    public function __construct()
    {
	require_once('config.php');
    	$this->config = $config;
    }
	
    //---------------------------------------------------------------------------------

    /**
    * Attempts to authenticate a user based on the creds provided.
    *
    * @return mixed void or string error message.
    */   
    public function do_login()
    {
        if ('POST' == $_SERVER['REQUEST_METHOD'])
        {
            $username = $this->_get_input('username');
            $password = $this->_get_input('password');
            if($this->_authenticate($username, $password))
            {
                session_start(); 
                $_SESSION['authenticated'] = 1; // store session data
                session_write_close();
                header('Location:'. $this->config['default_page']);
            }
            else
            {
                return $this->config['error_message'];
            }	
        }
		return false;
    }
    
    //---------------------------------------------------------------------------------

    /**
    * Private method which checks the credentials.
    * 
    * @param string $username
    * @param string $password;
    * @return boolean True if the user has entered the correct credentials, false by default.
    */
    private function _authenticate($username, $password)
    {
        if ($this->config['username'] == $username AND $this->config['password'] == sha1($password))
        {
            return TRUE;
        }
        return FALSE;
    }
     
    //--------------------------------------------------------------------------------- 

    /**
    * Gets the input values and cleans them for any unwanted chars.
    *
    * @param string $field_name The name of the field we want to get the posted values from.
    * @return string;
    */
    private function _get_input($field_name)
    {
        return htmlspecialchars($_POST[$field_name]);
    }
    
    //---------------------------------------------------------------------------------

    /**
    *  Checks to see if a user is logged in.
    *
    * @return boolean True if logged in, false by default.
    */
    public function is_logged_in()
    {
        if (isset($_SESSION['authenticated']))
        {
        	return$_SESSION['authenticated'];
    	}
        return FALSE;
    }
	
    //---------------------------------------------------------------------------------
	
    /**
    *  Generates a hashed password from a string.
    *
    * @param string $password
    * @return string
    */
    public function generate_password($password)
    {
       $code = '<h1>Copy and paste this line in the config file.</h1>';
	   $code .= '$config[\'password\'] = \''. sha1($password) .'\'; // '. $password;
	   return $code;
    }
	
    //---------------------------------------------------------------------------------
	
    /**
    *  Performs a logour operation by setting the session to an empty array and then
    *  destroying the session altogether. Once completed it returns the logout message
    *  located in the configuration. 
    *
    * @param string $password
    * @return string
    */
    public function do_logout()
    {
        session_start(); 
        $_SESSION = array();
        session_destroy(); 
        return $this->config['logout_message'];
    }
}

//Create new instance of the Login class
$login = new Login();

//Run the do_login method
$message = $login->do_login();